1. Field of the Invention
The present invention relates to photoelectric conversion elements and photoelectric conversion apparatus, and more particularly to photoelectric conversion elements that can perform a reset operation without operating an amplifying portion, and photoelectric conversion apparatus that can perform a high-speed reset operation.
2. Related Background Art
The conventional photoelectric conversion elements of an amplification type utilizing transistors, proposed in order to enhance the sensitivity of photoelectric conversion apparatus (including solid state image sensing devices etc.), include MOS type (normally, depletion MOS type) devices, bipolar type devices, and junction field effect transistor (JFET) type devices. In these photoelectric conversion elements, when light impinges on a MOS diode (of the MOS type) or on a pn junction diode (of the bipolar type or the JFET type), which is a part of constituent elements forming a photoelectric conversion element, the incident light is photoelectrically converted into a charge according thereto, the charge is stored, a signal according to the stored charge is amplified (in current amplification or in charge amplification), and then the amplified signal is output.
Among the above photoelectric conversion elements some photoelectric conversion elements are arranged to perform all operations including the photoelectric conversion operation, amplification operation, and initialization operation with a single transistor (which means that a photoelectric conversion element is composed of a single transistor). The photoelectric conversion elements of this type have two significant problems. Here, the initialization operation means an operation for setting the potential of a control region of the transistor to a certain reference value or an operation to completely deplete the control region. The control region of the transistor is a region for controlling the current, for example, which is a gate diffusion region in the JFET or a base diffusion region in the bipolar transistor.
The first problem is an increase of noise in the photoelectric conversion portion. For example, in the case of the MOS type device, photoelectric conversion is normally provided by a MOS diode with a gate electrode of polysilicon. In this case, since the silicon surface side is depleted at that point, it is directly affected by a great dark current appearing on the surface, resulting in increasing the noise. It also had the problem of a low utilization factor (quantum efficiency) of light because polysilicon has low transmittance of light.
The bipolar type and JFET type devices effecting photoelectric conversion by the pn junction diode are also affected by the dark current. This is because an ideal diode structure such as a buried photodiode suitably employed in a CCD image pickup element or the like cannot be realized because of the restriction that a part of the constituent elements of the transistor is utilized (which means that, in the case of the bipolar type and JFET type devices, a depletion layer occurring from the pn junction portion reaches the surface). Therefore, the noise becomes great because of the dark current.
Normally, these pn junction diodes perform such reset operation as to recombine the charge generated and stored, by transient and considerably deep forward bias drive by capacitive coupling. However, this reset method will cause the problem of occurrence of reset noise and after-image (lag).
A further problem is that when the charge generated and stored was reset and when blooming (bleeding) suppressing operation was carried out, the transistor also operated (or became "on"), and a large current flowed in the transistor itself constituting the photoelectric conversion element, which greatly changed the bias point (operating point) of the transistor transiently to change the amplification factor. For example, when a photoelectric conversion apparatus is composed of a lot of such photoelectric conversion elements arrayed, there occur variations in outputs from the photoelectric conversion elements, causing problems of the lowering the performance of apparatus (for example, S/N ratios) and increasing dissipation power because of the many arrayed elements.
The second problem is that the sensitivity is limited. To begin with, the above various (MOS type, bipolar type, and JFET type) transistors (photoelectric conversion elements) utilize a potential change caused when the charge generated by photoelectric conversion is stored in the control region in a floating state, in order to effect current amplification or charge amplification. Namely, they obtain an amplified output by utilizing a change of the surface potential of silicon under the gate electrode, in the case of the depletion MOS type transistor, or a potential change of the base region in the case of the bipolar device or of the gate region, in the case of the JFET type device.
Accordingly, in order to achieve high sensitivity, it is necessary to increase an amount of this potential change (stored charge amount/capacitance). For that purpose, the capacitance of the control region is preferably as small as possible. However, the area of the photoelectric conversion portion (a light-receiving aperture ratio) needs to be increased in order to raise the utilization factor of incident light and thereby increase the charge amount. However, in the case of the photoelectric conversion element where only one transistor performs the all operations (including the photoelectric conversion operation, the amplification operation, and the initialization operation), the control region is nothing but the photoelectric conversion portion, and, therefore, the capacitance becomes greater with an increase of the aperture ratio. As a result, the sensitivity is limited.
Also proposed on the other hand are photoelectric conversion elements arranged in such a manner that the photoelectric conversion portion is separated from an amplifying transistor, the charge generated and stored in the photoelectric conversion portion is transferred through a transfer gate of a transfer control portion to the control region of the transistor, and an output is obtained by current amplification or charge amplification. For example, Japanese Patent Laid-open Nos. 5-235317 (corresponding to U.S. patent application Ser. No. 08/261,135) and 5-275670 disclose photoelectric conversion elements in which the amplifying portion of the depletion type MOS transistor or the JFET is combined with the photodiode and the transfer control portion (transfer gate).
In the photoelectric conversion elements arranged by separating the photoelectric conversion portion from the amplifying transistor and providing the transfer gate, as described above, if a buried photodiode is used for the photoelectric conversion portion, the photoelectric conversion elements can be achieved with high quantum efficiency and without occurrence of lag, dark current, or reset noise.
When a buried photodiode in a vertical overflow structure is used for the photoelectric conversion portion, the blooming suppressing operation by the amplifying transistor becomes unnecessary, because the photodiode has a blooming suppressing function. For example, when a photoelectric conversion apparatus is composed of such photoelectric conversion elements, the apparatus is free of the problem of increase of dissipation power and the problem that variations appear in outputs from the photoelectric conversion elements due to changes of bias points (operating points).
Further, because the photoelectric conversion portion is separated from the amplifying transistor, the structure and size of the transistor can be optimized by taking only the amplifying function into consideration. Therefore, high sensitivity can be secured by decreasing the capacitance of the control region.
In addition, the new problems including the dark current, lag, and reset noise, caused by the transistor itself, can effectively be removed by the configuration and drive method of the photoelectric conversion apparatus with these photoelectric conversion elements arranged in a matrix.
Thus, the photoelectric conversion element with the separate photoelectric conversion portion and amplifying transistor and with the transfer gate is considerably lowered in noise and enhanced in sensitivity, as compared with the photoelectric conversion element arranged to perform all operations (including the photoelectric conversion operation, the amplification operation, and the initialization operation) by a transistor.
However, the above conventional photoelectric conversion element (which is the photoelectric conversion element provided with the separate photoelectric conversion portion and amplifying transistor and the transfer gate) had the problem that there is no improvement in the reset operation compared to the other conventional photoelectric conversion element arranged to perform the all operations by a single transistor.
Namely, the conventional photoelectric conversion element (the photoelectric conversion element with the separate photoelectric conversion portion and amplifying transistor and the transfer gate) also had the problem that when the reset operation was started in order to initialize the control region of the transistor, the amplifying transistor itself also operated (or became "on") at the same time therewith.
As a result, a large current flows in the amplifying transistor, which greatly changes the bias point (operating point) of the amplifying transistor transiently, thereby changing the amplification factor. For example, when a photoelectric conversion apparatus was composed of a lot of photoelectric conversion elements of this type arrayed, there were problems that variations appeared in outputs from the photoelectric conversion elements, that the performance of the apparatus (for example, S/N ratios) was degraded, and that the dissipation power increased because of the array of many elements.